Stage 1? I would assume they would just tweak the cars ecu, I had a M3 re mapped by a reputable tuning firm and the gains weren't noticeable.. 15nm and 10-20 bhp is hardly going to make a noticeable difference.
Mapping gains are Imo only worth your while on a turbo car and of course s/c cars pullys/belts up graded cooling systems etc..possibly head work on a n/a car but a lot of money for not much gain for your £, $ or €!